1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method, and non-transitory computer readable medium. Particularly, the present invention relates to a technique of performing exposure control processing for a white thin line on a black background in resolution conversion processing for outputting a high-resolution binary image signal from a low-resolution image output apparatus.
2. Description of the Related Art
When an image processing apparatus such as a printer, copying machine, or MFP (Multi Function Peripheral) prints an outline character, the white thin line sometimes becomes thinner or is disconnected by the background color. In particular, it may become difficult to read a small-size outline character.
There has conventionally been proposed a method of performing expansion processing for a white thin line part when a white thin line exists on a black background. In Japanese Patent Laid-Open No. 2011-254271, an image processing apparatus includes a detection unit which detects a character edge based on tag data representing the attribute of image data, and a determination unit which determines whether the character edge is the outer edge part of an outline character. Expansion processing is performed for a character whose character edge is determined to be the outer edge of an outline character.
In Japanese Patent Laid-Open No. 2012-84993, when priority is given to the reproducibility of a white thin line, a tone value at which the thickness of a white thin line in a dot screen corresponding to a tone value becomes smaller than a minimum line width reproducible when attention is paid to the thickness of a white thin line in an image forming apparatus is set as a threshold in the region of low tone values. If the tone value of an input image exceeds the threshold, the dot screen is switched to a line screen. In the region of high tone values, a tone value at which the thickness of a white thin line in a line screen corresponding to a tone value becomes smaller than a reproducible minimum line width is set as a threshold. If the tone value of an input image exceeds the threshold, the line screen is switched to an outline dot screen.
There has conventionally been proposed an arrangement in which a low-resolution image output apparatus outputs a high-resolution image signal as high-resolution image data in image processing by a printer, copying machine, MFP, or the like.
When converting the resolution of the first image data having a first resolution N into the second image data having a second resolution M lower than N, the resolution conversion ratio may not be an integer multiple. In this case, if a white thin line is emphasized by the conventional arrangement, the thickness of the white thin line sometimes changes upon outputting the second image data, depending on the position of the white thin line in the first image data.
In some cases, the resolution of the second image data after resolution conversion is different between the main scanning direction and the sub-scanning direction. In this case, if a white thin line is emphasized by the above arrangement, the thickness of a white thin line may differ between the main scanning direction and the sub-scanning direction upon outputting the second image data.